DESCRIPTION (Verbatim from Applicant's Abstract): Clinical isolates of the pathogenic yeast Candida albicans exhibit extensive variation in electrophoretic karyotypes and in phenotypic polymorphism. In this connection, systematic studies conducted in our laboratory revealed that laboratory strains of C. albicans spontaneously give rise to high frequencies of many different types of mutants having altered phenotypes and karyotypes. The significance of the chromosomal alterations was established with spontaneous mutants that acquired the ability to utilize alternative carbon sources. A causal relationship was established with a series of Sou- to Sou+ to Sou- to Sou+ derivatives, in which the Sou- (L-sorbose none-utilizing) and Sou+ (L-sorbose utilizing) strains were, respectively, disomic and monosomic for chromosome 5. Furthermore, transcription of the SOU1 gene, required for L-sorbose utilization, was regulated by the copy number of chromosome 5, in spite of the fact that SOU1 resides on a different chromosome. A hypothetical negative regulator, CSU51, was postulated to reside on chromosome 5, such that transcription of SOU1 is dependent on the ratio of the CSU51 to SOU1 copy number. Other examples of negative regulation by chromosome copy number include the utilization of D-arabinose, Aru- to Aru+, and resistance to the antifungal agent, fluconazole, FluS to FluR, thus establishing a general regulatory mechanism. The major long-term goal of the proposed research is to determine mechanisms of this newly-discovered regulatory process in C. albicans, by which gene expression is controlled by chromosome copy number. Several candidates of the negative regulator residing on chromosome 5 have been isolated from a library of chromosome 5 DNA, and these are being characterized. These regulators will be investigated for their direct or indirect interaction with the SOU1 structural gene. The additional negative regulators, which were retrieved from a total genomic library, and which are located on different chromosomes, will be analyzed for their involvement in the regulatory network controlled by chromosome copy number. This work establishes for the first time a negative regulatory network for a secondary carbon source in an important pathogen.